UNIX for Dummies Questions & Answers

Is there any way I could run two commands at the same time? Say I have in my script a command that grep a keyword from a huge size file:

zgrep $KEYWORD $FILE

and because this is a large file it takes a while to finish, so I would want that while zgrep is doing its job, I have a function that serves a progress bar:

Grepping..

Grepping....

This means that the dot (.) increases simultaneously while zgrep is doing its job internally in my script. Is that possible?

Run the command at background with &.

You didn't say which shell you were using, but something like this should work:

Code:

grep $PAT $FILE > foo.bar &
typeset PID=$!
echo "Processing...\c"
while ps $PID > /dev/null 2>&1; do
  echo ".\c" && sleep 1
done
echo "Done."

The "\c" in the echo command stops the CR/LF (ksh). Other shells probably use "echo -n" instead.

Of course, it just occurred to me that this is an endless loop if the background command never dies. And it's never good to program endless loops. Sooooooo:

Code:

grep $PAT $FILE > foo.bar &
typeset PID=$!
echo "Processing...\c"
typeset START_TIME=$SECONDS  TIMEOUT=1000
while ps $PID > /dev/null 2>&1; do
  echo ".\c" && sleep 1
  if [[ $(((SECONDS-START_TIME))) -gt $TIMEOUT ]]; then
    echo "TIMEOUT! ($TIMEOUT seconds)"
    break
  fi
done
echo "Done."

This way, the loop will exit after 1000 seconds regardless. This probably isn't much of an issue for "grep", but it's a good practice.

But what if the zgrep is not yet done on its job after the 1000 seconds?

I notice that the loop is still infinite.... while in progress I press CTRL+C to quit the script. Then I found out that foo.bar was created almost 2mins ago already.. but still the progress is still running, see below:


Orbix_BOX:/tmp> ./testScript
Mon Nov 26 08:06:44 PST 2007
Processing.......................................................................................... .....................................(CTRL+C)Orbix_BOX:/tmp> date
Mon Nov 26 08:08:57 PST 2007
Orbix_BOX:/tmp> ls -latr foo.bar
-rw-rw---- 1 orbix orbix 137065 Nov 26 08:06 foo.bar
Orbix_BOX:/tmp>

That's because the grep was running in background. If you want to kill it, for instance, you may want to use "trap" to kill it when you press CTRL+C:

Code:

[...]

function ctrlc {
   kill $GREP_PID
}

trap ctrlc INT

[...]

grep $PAT $FILE > foo.bar &
GREP_PID=$!

[...]

Regards.